Coronal Structure of Low-Mass Stars

TL;DR

This study examines how magnetic topology changes in low-mass stars across the fully-convective boundary and its impact on coronal properties like X-ray emission and stellar wind, using magnetic field reconstructions and coronal modeling.

Contribution

It provides new insights into the magnetic field geometry of low-mass stars and its influence on coronal emissions, especially across the fully-convective boundary.

Findings

Dipole component strength increases with decreasing stellar mass.

Open magnetic flux correlates with total magnetic flux.

X-ray saturation explained by coronal modeling and observed densities.

Abstract

We investigate the change in stellar magnetic topology across the fully-convective boundary and its effects on coronal properties. We consider both the magnitude of the open flux that influences angular momentum loss in the stellar wind and X-ray emission measure. We use reconstructed maps of the radial magnetic field at the stellar surface and the potential-field source surface method to extrapolate a 3D coronal magnetic field for a sample of early-to-mid M dwarfs. During the magnetic reconstruction process it is possible to force a solution towards field geometries that are symmetric or antisymmetric about the equator but we demonstrate that this has only a modest impact on the coronal tracers mentioned above. We find that the dipole component of the field, which governs the large-scale structure, becomes increasingly strong as the stellar mass decreases, while the magnitude of the open (wind-bearing) magnetic flux is proportional to the magnitude of the reconstructed magnetic flux. By assuming a hydrostatic and isothermal corona we calculate X-ray emission measures (in magnitude and rotational modulation) for each star and, using observed stellar densities as a constraint, we reproduce the observed X-ray saturation at Ro < 0.1. We find that X-ray rotational modulation is not a good indicator of magnetic structure as it shows no trend with Rossby number but can be useful in discriminating between different assumptions on the field geometry.